Since the end of the 19th century until now, small capacity vapor compression systems have been used for most cooling applications, rather than absorption cooling systems due to their higher performance per unit cost. Absorption cooling systems trace their origins to the 1700's when it was observed that ice could be produced by evaporation of pure water from a vessel contained within an evacuated container in the presence of sulfuric acid. As early as 1810, ice was made by connecting a vessel containing water to a second vessel containing sulfuric acid, where by the absorption of water vapor by acid, layers of ice were formed on the water surface in the first vessel. In 1859, Ferdinand Carre introduced a refrigeration machine using water/ammonia as the working fluid for making ice and for food storage. In the 1950's, a system using lithium bromide/water as the working fluid was introduced for industrial absorption refrigeration systems (ARSs), as illustrated in FIG. 1.
In 1956, a double-effect absorption system, as illustrated in FIG. 2, was introduced and remains an industrial standard for a high performance heat-operated refrigeration cycle. A double-effect absorption refrigeration cycle supplies high temperature heat from an external source to a first-effect generator to liberate vapor refrigerant from the solution, which is condensed at high pressure in a second-effect generator where the heat rejected is used to generate addition refrigerant vapor from the concentrated solution coming from the first-effect generator. Such a configuration is considered to be a series-flow-double-effect absorption system and is a combination of two single-effect absorption systems. A single-effect absorption system is characterized by its coefficient of performance (COP). The COPsingle is the cooling effect produced from the refrigerant generated from the first-effect generator per unit of heat inputted from an external source. In a single-effect absorption system, the heat rejected from the condenser is approximately equal to the cooling capacity obtained; therefore, the heat supplied to the second generator is about equal to COPsingle. The cooling effect produced from the second-effect generator is about COPsingle2 and, therefore, the COP of this double-effect absorption system is COPdouble=COPsingle+COPsingle2. Accordingly, a double effect absorption system has a COPdouble of about 1.2 when the corresponding single-effect system has a COP of about 0.7.
Large-scale cooling capacity ARSs have been used for many years, and only recently, driven by the goal of extracting as much energy as possible from systems where heat is generated yet cooling is desired, small-scale absorption chillers having cooling capacities of 10-15 kW have been available. However, these units generally employ shell and tube absorbers and weigh about 400 kg or more, and hence are generally restricted to cooling buildings with sufficient room for the ARS. The small-scale ARS can use waste heat from industrial or residential heating sources when available, or the ARS can employ solar heaters.
There remains a need to develop more compact and less expensive solar and waste heat powered ARSs with substantially greater efficiency than the existing technology has permitted. An absorber using a permeable membrane with a plate and frame absorber was modeled in Ali, Applied Energy 2010, 87, 1112-21. Refrigerant channel thicknesses of 1 to 4 mm were modeled, with almost no effect on heat and mass transfer areas required in the absorber. When compared to a conventional absorber, the plate and frame absorber of Ali, at its design point limits, required 2.5% greater mass transfer area but 42.7% lower heat transfer area for equivalent efficiencies, and therefore did not suggest a significant improvement in ARS size. Hence, ARS architecture with enhanced heat and mass transport processes that allow an enhancement in performance and reliability at a significantly reduced size and cost remains to be realized.